The present invention relates to the preparation of cyanopyridines. Cyanopyridines are useful as intermediates in the manufacture of useful compounds. In U.S. Pat. No. 4,341,907, cyanopyridine serves a catalytic role. In U.S. Pat. No. 5,719,045, the preparation of cyanopyridine by ammoxidation from picoline (also known as methylpyridine) is disclosed. Cyanopyridines also find application as a building block for the preparation of other compositions, including crop protection chemicals such as the systemic broadleaf herbicides picloram (4-amino-3,5,6-trichloro-2-pyridine carboxylic acid) and aminopyralid (4-amino-3,6-dichloro-2-pyridine carboxylic acid).
In the production of cyanopyridine from picoline, aqueous quench fluids provide a convenient means to quench gaseous reaction mixtures containing cyanopyridine reaction product. The use of aqueous quench fluids, however, generates hydrolysis by-products. The present invention reduces hydrolysis by-products.
The preparation of substituted heterocyclic compounds is often conducted at elevated temperatures with gaseous reactants over a solid catalyst. Similarly, removal of substituents from heterocyclic compounds is also often conducted at elevated temperatures with gaseous reactants over solid catalysts. Typical of reactions that remove substituents from heterocyclic compounds is that disclosed in U.S. Pat. No. 3,689,491 concerning the oxidation of 3-picoline (3-methylpyridine or β-picoline) over vanadium pentoxide/titanium dioxide catalyst to produce pyridine. The reaction is said to be operated between 300° C. and 380° C. resulting in gaseous reaction product at that temperature. Gaseous reaction product exiting the reactor is quenched with cold water in a scrubber.
UK 790,937 describes the recovery of cyanopyridines from ammoxidation reactions involving picolines with ammonia and oxygen. A recycle of water including the water soluble reaction products is used to quench the gaseous reaction product containing cyanopyridine as it exits from the reactor. The cyanopyridine is subsequently separated from the quench water by cooling the water prior to recycle to the quencher. The recycled quench liquid includes by-products nicotinamide, cyanide, and CO2.
U.S. Pat. No. 4,810,794 discloses absorbing gaseous reaction product including pyridine in water.
Cyanopyridines are manufactured by ammoxidation of picolines, as reactor feed. Ammoxidation reactions operate in temperature ranges from 300° C. to 450° C. Oxygen and ammonia in the presence of catalysts react with picolines to form cyanopyridines at high reaction temperatures. As may be expected, at lower temperatures reactions may be slowed to an uneconomical rate whereas as temperatures elevate the reaction generates increasing amounts of unwanted by-products. An economical operating temperature will balance reaction rate with desired reaction products.
For a typical aqueous quench in a picoline ammoxidation reaction, the gaseous reaction products are quenched from the reaction temperature to a temperature convenient for work up of reaction products, such as 50° C., in the quench operation. The aqueous quench fluid is fed to the quench operation at a sufficiently lower temperature to cool the reaction products to a temperature convenient for work up. As necessary, temperature adjustment of aqueous quench fluid is provided.
Typically, an organic extraction fluid, such as benzene, is useful to remove the cyanopyridine reaction product into the organic phase. The aqueous extraction fluid is recycled to the aqueous quench operation and added to necessary make up water. The organic phase benzene and cyanopyridine stream may be separated by distillation followed by work-up and purification of the product cyanopyridine. The organic extractant may be returned to the extraction step.
Hydrolysis of cyanopyridines can lead to pyridinecarboxamide (also known as picolinamide). It is thought that by-products such as pyridinecarboxamide result from hydrolysis of the desired cyanopyridine, a nitrile, in the presence of water and ammonia in the quench liquid. It would be desirable to recover cyanopyridine reaction products without generating hydrolysis by-products. Hydrolysis of pyridinecarboxamide, a hydrolysis product of cyanopyridine, can further lead to pyridine-2-carboxylic acid formation.